<!DOCTYPE html>
<html lang="zh" dir="ltr" class="client-nojs">
<head>
<title>折叠表达式(C++17 起)</title>
<meta charset="UTF-8">
<meta name="generator" content="MediaWiki 1.21.2">
<link rel="shortcut icon" href="../../../common/favicon.ico">
<link rel="stylesheet" href="../../../common/ext.css">
<meta name="ResourceLoaderDynamicStyles" content="">
<link rel="stylesheet" href="../../../common/site_modules.css">
<style>a:lang(ar),a:lang(ckb),a:lang(fa),a:lang(kk-arab),a:lang(mzn),a:lang(ps),a:lang(ur){text-decoration:none}#toc{display:none}.editsection{display:none}
/* cache key: mwiki1-mwiki_zh_:resourceloader:filter:minify-css:7:15cea3ec788a65b5187d4018eed543bf */</style>

<script src="../../../common/startup_scripts.js"></script>
<script>if(window.mw){
mw.config.set({"wgCanonicalNamespace":"","wgCanonicalSpecialPageName":false,"wgNamespaceNumber":0,"wgPageName":"cpp/language/fold","wgTitle":"cpp/language/fold","wgCurRevisionId":60316,"wgArticleId":9251,"wgIsArticle":true,"wgAction":"view","wgUserName":null,"wgUserGroups":["*"],"wgCategories":[],"wgBreakFrames":false,"wgPageContentLanguage":"zh","wgSeparatorTransformTable":["",""],"wgDigitTransformTable":["",""],"wgDefaultDateFormat":"zh","wgMonthNames":["","1月","2月","3月","4月","5月","6月","7月","8月","9月","10月","11月","12月"],"wgMonthNamesShort":["","1月","2月","3月","4月","5月","6月","7月","8月","9月","10月","11月","12月"],"wgRelevantPageName":"cpp/language/fold","wgUserVariant":"zh","wgRestrictionEdit":[],"wgRestrictionMove":[]});
}</script><script>if(window.mw){
mw.loader.implement("user.options",function(){mw.user.options.set({"ccmeonemails":0,"cols":80,"date":"default","diffonly":0,"disablemail":0,"disablesuggest":0,"editfont":"default","editondblclick":0,"editsection":0,"editsectiononrightclick":0,"enotifminoredits":0,"enotifrevealaddr":0,"enotifusertalkpages":1,"enotifwatchlistpages":0,"extendwatchlist":0,"externaldiff":0,"externaleditor":0,"fancysig":0,"forceeditsummary":0,"gender":"unknown","hideminor":0,"hidepatrolled":0,"imagesize":2,"justify":0,"math":1,"minordefault":0,"newpageshidepatrolled":0,"nocache":0,"noconvertlink":0,"norollbackdiff":0,"numberheadings":0,"previewonfirst":0,"previewontop":1,"quickbar":5,"rcdays":7,"rclimit":50,"rememberpassword":0,"rows":25,"searchlimit":20,"showhiddencats":0,"showjumplinks":1,"shownumberswatching":1,"showtoc":0,"showtoolbar":1,"skin":"cppreference2","stubthreshold":0,"thumbsize":2,"underline":2,"uselivepreview":0,"usenewrc":0,"watchcreations":0,"watchdefault":0,"watchdeletion":0,
"watchlistdays":3,"watchlisthideanons":0,"watchlisthidebots":0,"watchlisthideliu":0,"watchlisthideminor":0,"watchlisthideown":0,"watchlisthidepatrolled":0,"watchmoves":0,"wllimit":250,"variant":"zh","language":"zh","searchNs0":true,"searchNs1":false,"searchNs2":false,"searchNs3":false,"searchNs4":false,"searchNs5":false,"searchNs6":false,"searchNs7":false,"searchNs8":false,"searchNs9":false,"searchNs10":false,"searchNs11":false,"searchNs12":false,"searchNs13":false,"searchNs14":false,"searchNs15":false,"gadget-MathJax":1,"gadget-ColiruCompiler":1});;},{},{});mw.loader.implement("user.tokens",function(){mw.user.tokens.set({"editToken":"+\\","patrolToken":false,"watchToken":false});;},{},{});
/* cache key: mwiki1-mwiki_zh_:resourceloader:filter:minify-js:7:258d7cd6aa9aa67dee25e01fb6a9e505 */
}</script>
<script>if(window.mw){
mw.loader.load(["mediawiki.page.startup","mediawiki.legacy.wikibits","mediawiki.legacy.ajax"]);
}</script>
<style type="text/css">/*<![CDATA[*/
.source-cpp {line-height: normal;}
.source-cpp li, .source-cpp pre {
	line-height: normal; border: 0px none white;
}
/**
 * GeSHi Dynamically Generated Stylesheet
 * --------------------------------------
 * Dynamically generated stylesheet for cpp
 * CSS class: source-cpp, CSS id: 
 * GeSHi (C) 2004 - 2007 Nigel McNie, 2007 - 2008 Benny Baumann
 * (http://qbnz.com/highlighter/ and http://geshi.org/)
 * --------------------------------------
 */
.cpp.source-cpp .de1, .cpp.source-cpp .de2 {font: normal normal 1em/1.2em monospace; margin:0; padding:0; background:none; vertical-align:top;}
.cpp.source-cpp  {font-family:monospace;}
.cpp.source-cpp .imp {font-weight: bold; color: red;}
.cpp.source-cpp li, .cpp.source-cpp .li1 {font-weight: normal; vertical-align:top;}
.cpp.source-cpp .ln {width:1px;text-align:right;margin:0;padding:0 2px;vertical-align:top;}
.cpp.source-cpp .li2 {font-weight: bold; vertical-align:top;}
.cpp.source-cpp .kw1 {color: #0000dd;}
.cpp.source-cpp .kw2 {color: #0000ff;}
.cpp.source-cpp .kw3 {color: #0000dd;}
.cpp.source-cpp .kw4 {color: #0000ff;}
.cpp.source-cpp .co1 {color: #909090;}
.cpp.source-cpp .co2 {color: #339900;}
.cpp.source-cpp .coMULTI {color: #ff0000; font-style: italic;}
.cpp.source-cpp .es0 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es1 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es2 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es3 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es4 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es5 {color: #008000; font-weight: bold;}
.cpp.source-cpp .br0 {color: #008000;}
.cpp.source-cpp .sy0 {color: #008000;}
.cpp.source-cpp .sy1 {color: #000080;}
.cpp.source-cpp .sy2 {color: #000040;}
.cpp.source-cpp .sy3 {color: #000040;}
.cpp.source-cpp .sy4 {color: #008080;}
.cpp.source-cpp .st0 {color: #008000;}
.cpp.source-cpp .nu0 {color: #000080;}
.cpp.source-cpp .nu6 {color: #000080;}
.cpp.source-cpp .nu8 {color: #000080;}
.cpp.source-cpp .nu12 {color: #000080;}
.cpp.source-cpp .nu16 {color:#000080;}
.cpp.source-cpp .nu17 {color:#000080;}
.cpp.source-cpp .nu18 {color:#000080;}
.cpp.source-cpp .nu19 {color:#000080;}
.cpp.source-cpp .ln-xtra, .cpp.source-cpp li.ln-xtra, .cpp.source-cpp div.ln-xtra {background-color: #ffc;}
.cpp.source-cpp span.xtra { display:block; }

/*]]>*/
</style><style type="text/css">/*<![CDATA[*/
.source-text {line-height: normal;}
.source-text li, .source-text pre {
	line-height: normal; border: 0px none white;
}
/**
 * GeSHi Dynamically Generated Stylesheet
 * --------------------------------------
 * Dynamically generated stylesheet for text
 * CSS class: source-text, CSS id: 
 * GeSHi (C) 2004 - 2007 Nigel McNie, 2007 - 2008 Benny Baumann
 * (http://qbnz.com/highlighter/ and http://geshi.org/)
 * --------------------------------------
 */
.text.source-text .de1, .text.source-text .de2 {font: normal normal 1em/1.2em monospace; margin:0; padding:0; background:none; vertical-align:top;}
.text.source-text  {font-family:monospace;}
.text.source-text .imp {font-weight: bold; color: red;}
.text.source-text li, .text.source-text .li1 {font-weight: normal; vertical-align:top;}
.text.source-text .ln {width:1px;text-align:right;margin:0;padding:0 2px;vertical-align:top;}
.text.source-text .li2 {font-weight: bold; vertical-align:top;}
.text.source-text .ln-xtra, .text.source-text li.ln-xtra, .text.source-text div.ln-xtra {background-color: #ffc;}
.text.source-text span.xtra { display:block; }

/*]]>*/
</style><!--[if lt IE 7]><style type="text/css">body{behavior:url("/mwiki/skins/cppreference2/csshover.min.htc")}</style><![endif]--></head>
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_fold skin-cppreference2 action-view cpp-navbar">
        <!-- header -->
        <!-- /header -->
        <!-- content -->
<div id="cpp-content-base">
            <div id="content">
                <a id="top"></a>
                <div id="mw-js-message" style="display:none;"></div>
                                <!-- firstHeading -->
<h1 id="firstHeading" class="firstHeading">折叠表达式<span class="t-mark-rev t-since-cxx17">(C++17 起)</span></h1>
                <!-- /firstHeading -->
                <!-- bodyContent -->
                <div id="bodyContent">
                                        <!-- tagline -->
                    <div id="siteSub">来自cppreference.com</div>
                    <!-- /tagline -->
                                        <!-- subtitle -->
                    <div id="contentSub"><span class="subpages">&lt; <a href="../../cpp.html" title="cpp">cpp</a>‎ | <a href="../language.html" title="cpp/language">language</a></span></div>
                    <!-- /subtitle -->
                                                            <!-- bodycontent -->
                    <div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr"><div class="t-navbar" style=""><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="../../cpp.html" title="cpp"> C++</a><div class="t-navbar-menu"><div><div><table class="t-nv-begin" cellpadding="0" style="line-height:1.1em;">
<tr class="t-nv"><td colspan="5"> <a href="../language.html" title="cpp/language">语言</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../header.html" title="cpp/header">标准库头文件</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../freestanding.html" title="cpp/freestanding"> 自立与有宿主实现</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../named_req.html" title="cpp/named req">具名要求</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../utility.html#.E8.AF.AD.E8.A8.80.E6.94.AF.E6.8C.81" title="cpp/utility">语言支持库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../concepts.html" title="cpp/concepts">概念库</a> <span class="t-mark-rev t-since-cxx20">(C++20)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../error.html" title="cpp/error">诊断库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../utility.html" title="cpp/utility">工具库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../string.html" title="cpp/string">字符串库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="https://zh.cppreference.com/w/cpp/container" title="cpp/container">容器库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../iterator.html" title="cpp/iterator">迭代器库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../ranges.html" title="cpp/ranges"> 范围库</a> <span class="t-mark-rev t-since-cxx20">(C++20)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../algorithm.html" title="cpp/algorithm">算法库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../numeric.html" title="cpp/numeric">数值库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../io.html" title="cpp/io">输入/输出库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../locale.html" title="cpp/locale">本地化库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../regex.html" title="cpp/regex">正则表达式库</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../atomic.html" title="cpp/atomic">原子操作库</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../thread.html" title="cpp/thread">线程支持库</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../filesystem.html" title="cpp/filesystem">文件系统库</a> <span class="t-mark-rev t-since-cxx17">(C++17)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../experimental.html" title="cpp/experimental">技术规范</a> </td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="../language.html" title="cpp/language">C++ 语言</a></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="templates.html" title="cpp/language/templates">模板</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="template_parameters.html" title="cpp/language/template parameters">形参与实参</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template">类模板</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template">函数模板</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="member_template.html" title="cpp/language/member template">类成员模板</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="variable_template.html" title="cpp/language/variable template">变量模板</a><span class="t-mark-rev t-since-cxx14">(C++14)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="template_argument_deduction.html" title="cpp/language/template argument deduction">模板实参推导</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class_template_argument_deduction.html" title="cpp/language/class template argument deduction">类模板实参推导</a><span class="t-mark-rev t-since-cxx17">(C++17)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization">显式（全）特化</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="partial_specialization.html" title="cpp/language/partial specialization">部分特化</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="dependent_name.html" title="cpp/language/dependent name">待决名</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="parameter_pack.html" title="cpp/language/parameter pack">形参包</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="sizeof....html" title="cpp/language/sizeof..."><span class="t-lines"><span>sizeof...</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink">折叠表达式</strong><span class="t-mark-rev t-since-cxx17">(C++17)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="sfinae.html" title="cpp/language/sfinae"> SFINAE</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="constraints.html" title="cpp/language/constraints">制约与概念</a><span class="t-mark-rev t-since-cxx20">(C++20)</span> </td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>以二元运算符对<a href="parameter_pack.html" title="cpp/language/parameter pack">形参包</a>进行规约（<a href="https://en.wikipedia.org/wiki/Fold_(higher-order_function)" class="extiw" title="enwiki:Fold (higher-order function)">折叠</a>）。
</p>
<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>(</b></code> <span class="t-spar">形参包</span> <span class="t-spar">op</span> <code><b>...</b></code> <code><b>)</b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>(</b></code> <code><b>...</b></code> <span class="t-spar">op</span> <span class="t-spar">形参包</span> <code><b>)</b></code>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>(</b></code> <span class="t-spar">形参包</span> <span class="t-spar">op</span> <code><b>...</b></code> <span class="t-spar">op</span> <span class="t-spar">初值</span> <code><b>)</b></code>
</td>
<td> (3)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>(</b></code> <span class="t-spar">初值</span> <span class="t-spar">op</span> <code><b>...</b></code> <span class="t-spar">op</span> <span class="t-spar">形参包</span> <code><b>)</b></code>
</td>
<td> (4)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> 一元右折叠</div>
<div class="t-li1"><span class="t-li">2)</span> 一元左折叠</div>
<div class="t-li1"><span class="t-li">3)</span> 二元右折叠</div>
<div class="t-li1"><span class="t-li">4)</span> 二元左折叠</div>
<table class="t-par-begin">


<tr class="t-par">
<td>  <span class="t-spar">op</span>
</td>
<td> -
</td>
<td>  任何下列 32 个<i>二元</i>运算符之一：<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">+</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">/</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">%</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">^</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&amp;</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">|</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&lt;</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&gt;</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&lt;&lt;</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&gt;&gt;</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">+</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">/</span><span class="sy1">=</span></span></span>  <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">%</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">^</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&amp;</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">|</span><span class="sy1">=</span></span></span>  <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&lt;&lt;=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&gt;&gt;=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">==</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&lt;=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">&gt;=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&amp;&amp;</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">||</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp">,</span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp">.<span class="sy2">*</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="sy1">&gt;</span><span class="sy2">*</span></span></span>。在二元折叠中，两个 <span class="t-spar">op</span> 必须相同。
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">形参包</span>
</td>
<td> -
</td>
<td>  含未展开的<a href="parameter_pack.html" title="cpp/language/parameter pack">形参包</a>且其顶层不含有<a href="operator_precedence.html" title="cpp/language/operator precedence">优先级</a>低于转型（正式而言，是 <span class="t-spar">转型表达式</span>）的运算符的表达式
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">初值</span>
</td>
<td> -
</td>
<td>  不含未展开的<a href="parameter_pack.html" title="cpp/language/parameter pack">形参包</a>且其顶层不含有<a href="operator_precedence.html" title="cpp/language/operator precedence">优先级</a>低于转型（正式而言，是 <span class="t-spar">转型表达式</span>）的运算符的表达式
</td></tr></table>
<p>注意开与闭括号是折叠表达式的一部分。
</p>
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
<p><i>折叠表达式</i>的实例化按如下方式展开成表达式 <code>e</code>：
</p>
<div class="t-li1"><span class="t-li">1)</span> 一元右折叠 <span class="texhtml" style="white-space: nowrap;">(E op ...)</span> 成为 <span class="texhtml" style="white-space: nowrap;">(E<span class="t-su t-su-b"><br>1</span> op (... op (E<span class="t-su t-su-b"><br>N-1</span> op E<span class="t-su t-su-b"><br>N</span>)))</span></div>
<div class="t-li1"><span class="t-li">2)</span> 一元左折叠 <span class="texhtml" style="white-space: nowrap;">(... op E)</span> 成为 <span class="texhtml" style="white-space: nowrap;">(((E<span class="t-su t-su-b"><br>1</span> op E<span class="t-su t-su-b"><br>2</span>) op ...) op E<span class="t-su t-su-b"><br>N</span>)</span></div>
<div class="t-li1"><span class="t-li">3)</span> 二元右折叠 <span class="texhtml" style="white-space: nowrap;">(E op ... op I)</span> 成为 <span class="texhtml" style="white-space: nowrap;">(E<span class="t-su t-su-b"><br>1</span> op (... op (E<span class="t-su t-su-b"><br>N−1</span> op (E<span class="t-su t-su-b"><br>N</span> op I))))</span></div>
<div class="t-li1"><span class="t-li">4)</span> 二元左折叠 <span class="texhtml" style="white-space: nowrap;">(I op ... op E)</span> 成为 <span class="texhtml" style="white-space: nowrap;">((((I op E<span class="t-su t-su-b"><br>1</span>) op E<span class="t-su t-su-b"><br>2</span>) op ...) op E<span class="t-su t-su-b"><br>N</span>)</span></div>
<p>（其中 <span class="texhtml" style="white-space: nowrap;">N</span> 是包展开中的元素数）
</p><p>例如， 
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span>... <span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw4">bool</span> all<span class="br0">(</span>Args... <span class="me1">args</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="br0">(</span>... <span class="sy3">&amp;&amp;</span> args<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
 
<span class="kw4">bool</span> b <span class="sy1">=</span> all<span class="br0">(</span><span class="kw2">true</span>, <span class="kw2">true</span>, <span class="kw2">true</span>, <span class="kw2">false</span><span class="br0">)</span><span class="sy4">;</span>
 <span class="co1">// 在 all() 中，一元左折叠展开成</span>
 <span class="co1">//  return ((true &amp;&amp; true) &amp;&amp; true) &amp;&amp; false;</span>
 <span class="co1">// b 为 false</span></pre></div></div>
<p>将一元折叠用于零长包展开时，仅允许下列运算符：
</p>
<div class="t-li1"><span class="t-li">1)</span> 逻辑与（<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&amp;&amp;</span></span></span>）。空包的值为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span></div>
<div class="t-li1"><span class="t-li">2)</span> 逻辑或（<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">||</span></span></span>）。空包的值为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span></div>
<div class="t-li1"><span class="t-li">3)</span> 逗号运算符（<span class="t-c"><span class="mw-geshi cpp source-cpp">,</span></span>）。空包的值为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span><span class="br0">(</span><span class="br0">)</span></span></span></div>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
<p>若用作 <span class="t-spar">初值</span> 或 <span class="t-spar">形参包</span> 的表达式在顶层具有优先级低于转型的运算符，则它可以加括号：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> ...<span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw4">int</span> sum<span class="br0">(</span>Args<span class="sy3">&amp;&amp;</span>... <span class="me1">args</span><span class="br0">)</span> <span class="br0">{</span>
<span class="co1">//    return (args + ... + 1 * 2); // 错误：优先级低于转型的运算符</span>
    <span class="kw1">return</span> <span class="br0">(</span>args <span class="sy2">+</span> ... <span class="sy2">+</span> <span class="br0">(</span><span class="nu0">1</span> <span class="sy2">*</span> <span class="nu0">2</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span>
<span class="br0">}</span></pre></div></div>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;vector&gt;</span>
<span class="co2">#include &lt;climits&gt;</span>
<span class="co2">#include &lt;cstdint&gt;</span>
<span class="co2">#include &lt;type_traits&gt;</span>
<span class="co2">#include &lt;utility&gt;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> ...<span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw4">void</span> printer<span class="br0">(</span>Args<span class="sy3">&amp;&amp;</span>... <span class="me1">args</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="br0">(</span><a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> ... <span class="sy1">&lt;&lt;</span> args<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw4">void</span> push_back_vec<span class="br0">(</span><a href="../container/vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span> v, Args<span class="sy3">&amp;&amp;</span>... <span class="me1">args</span><span class="br0">)</span>
<span class="br0">{</span>
    static_assert<span class="br0">(</span><span class="br0">(</span><a href="http://zh.cppreference.com/w/cpp/types/is_constructible"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1">&lt;</span>T, Args<span class="sy3">&amp;</span><span class="sy1">&gt;</span> <span class="sy3">&amp;&amp;</span> ...<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="br0">(</span>v.<span class="me1">push_back</span><span class="br0">(</span>args<span class="br0">)</span>, ...<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// 基于 http://stackoverflow.com/a/36937049 的编译时端序交换 </span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a>... <span class="me1">N</span><span class="sy1">&gt;</span>
<span class="kw4">constexpr</span> T bswap_impl<span class="br0">(</span>T i, <a href="../utility/integer_sequence.html"><span class="kw1108">std::<span class="me2">index_sequence</span></span></a><span class="sy1">&lt;</span>N...<span class="sy1">&gt;</span><span class="br0">)</span> <span class="br0">{</span>
  <span class="kw1">return</span> <span class="br0">(</span><span class="br0">(</span><span class="br0">(</span>i <span class="sy1">&gt;&gt;</span> N<span class="sy2">*</span><a href="../types/climits.html"><span class="kw387">CHAR_BIT</span></a> <span class="sy3">&amp;</span> <a href="../types/integer.html"><span class="kw120">std::<span class="me2">uint8_t</span></span></a><span class="br0">(</span><span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="br0">(</span>sizeof<span class="br0">(</span>T<span class="br0">)</span><span class="sy2">-</span><span class="nu0">1</span><span class="sy2">-</span>N<span class="br0">)</span><span class="sy2">*</span><a href="../types/climits.html"><span class="kw387">CHAR_BIT</span></a><span class="br0">)</span> <span class="sy3">|</span> ...<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> U <span class="sy1">=</span> <a href="../types/make_unsigned.html"><span class="kw619">std::<span class="me2">make_unsigned_t</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;&gt;</span>
<span class="kw4">constexpr</span> U bswap<span class="br0">(</span>T i<span class="br0">)</span> <span class="br0">{</span>
  <span class="kw1">return</span> bswap_impl<span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="br0">(</span>i, <a href="../utility/integer_sequence.html"><span class="kw1110">std::<span class="me2">make_index_sequence</span></span></a><span class="sy1">&lt;</span>sizeof<span class="br0">(</span>T<span class="br0">)</span><span class="sy1">&gt;</span><span class="br0">{</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    printer<span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span>, <span class="st0">"abc"</span><span class="br0">)</span><span class="sy4">;</span>
 
    <a href="../container/vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> v<span class="sy4">;</span>
    push_back_vec<span class="br0">(</span>v, <span class="nu0">6</span>, <span class="nu0">2</span>, <span class="nu0">45</span>, <span class="nu0">12</span><span class="br0">)</span><span class="sy4">;</span>
    push_back_vec<span class="br0">(</span>v, <span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">9</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i <span class="sy4">:</span> v<span class="br0">)</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> i <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span><span class="sy4">;</span>
 
    static_assert<span class="br0">(</span>bswap<span class="sy1">&lt;</span><a href="../types/integer.html"><span class="kw121">std::<span class="me2">uint16_t</span></span></a><span class="sy1">&gt;</span><span class="br0">(</span>0x1234u<span class="br0">)</span><span class="sy1">==</span>0x3412u<span class="br0">)</span><span class="sy4">;</span>
    static_assert<span class="br0">(</span>bswap<span class="sy1">&lt;</span><a href="../types/integer.html"><span class="kw123">std::<span class="me2">uint64_t</span></span></a><span class="sy1">&gt;</span><span class="br0">(</span>0x0123456789abcdefULL<span class="br0">)</span><span class="sy1">==</span>0xefcdab8967452301ULL<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">123abc
6 2 45 12 1 2 9</pre></div></div> 
</div>

<!-- 
NewPP limit report
Preprocessor visited node count: 2376/1000000
Preprocessor generated node count: 6098/1000000
Post‐expand include size: 45833/2097152 bytes
Template argument size: 12658/2097152 bytes
Highest expansion depth: 15/40
Expensive parser function count: 0/100
-->

<!-- Saved in parser cache with key mwiki1-mwiki_zh_:pcache:idhash:9251-0!*!0!!zh!*!zh!* and timestamp 20190831055822 -->
</div>                    <!-- /bodycontent -->
                                        <!-- printfooter -->
                    <div class="printfooter">
                    来自“<a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/language/fold&amp;oldid=60316">https://zh.cppreference.com/mwiki/index.php?title=cpp/language/fold&amp;oldid=60316</a>”                    </div>
                    <!-- /printfooter -->
                                                            <!-- catlinks -->
                    <!-- /catlinks -->
                                                            <div class="visualClear"></div>
                    <!-- debughtml -->
                                        <!-- /debughtml -->
                </div>
                <!-- /bodyContent -->
            </div>
        </div>
        <!-- /content -->
        <!-- footer -->
        <div id="cpp-footer-base" class="noprint">
            <div id="footer">
                        <div id="cpp-navigation">
            <h5>导航</h5>
            <ul><li><a href="https://zh.cppreference.com/w/cpp/language/fold">Online version</a></li><li>Offline version retrieved 2020-03-14 18:00.</li></ul></div>
                        <ul id="footer-info">
                                    <li id="footer-info-lastmod"> 本页面最后修改于2019年5月13日 (星期一) 02:35。</li>
                                    <li id="footer-info-viewcount">此页面已被浏览过5,834次。</li>
                            </ul>
                    </div>
        </div>
        <!-- /footer -->
        <script>if(window.mw){
mw.loader.state({"site":"loading","user":"missing","user.groups":"ready"});
}</script>
<script src="../../../common/skin_scripts.js"></script>
<script>if(window.mw){
mw.loader.load(["mediawiki.action.view.postEdit","mediawiki.user","mediawiki.page.ready","mediawiki.searchSuggest","mediawiki.hidpi","ext.gadget.MathJax","ext.gadget.ColiruCompiler"], null, true);
}</script>
<script src="../../../common/site_scripts.js"></script>
<script type="text/javascript">
var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-2828341-1']);
_gaq.push(['_setDomainName', 'cppreference.com']);
_gaq.push(['_trackPageview']);
</script><!-- Served in 2.217 secs. -->
	</body>
<!-- Cached 20190831055822 -->
</html>